<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <title>利用边框描绘三角形</title>
    <style>
        div {
            width: 0;
            border-bottom: 10px solid black;
            /* transparent 透明 */
            border-left: 10px solid transparent;
            border-right: 10px solid transparent;
        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>